Vehicles are employing increasingly greater numbers of functional options. Telematics, entertainment, information, navigation, collision avoidance, vehicle stability, emission control, steer by-wire, and electronic braking are but a few examples. Consequently, the use of electronic vehicle control modules has proliferated. This is a trend which promises to continue. Vehicle control modules may continue to actively operate systems even after the vehicle has been shut down and is inactive (key-off). For example, various lighting controls may remain active to enable various courtesy lighting features. Certain telematics and keyless entry systems are required to be active for receiving signals, for example to allow for remotely unlocking the vehicle. Also, certain emission diagnostic systems may remain active to perform various functional tests which can only be performed accurately after a period of vehicle inactivity. Apart from such key-off functions, all control modules have an associated current draw for maintaining associated volatile memory devices. Moreover, control modules and other vehicle electrical components may have associated leakage currents that are nothing more than parasitic draws on the battery. If unchecked, such draws will eventually result in depletion of the vehicle battery below an acceptable state of charge. The time to depletion is becoming less and less as the various key-off loads become greater.
It is known to monitor current draw from a vehicle battery or battery state of charge during key-off periods. Such monitoring is performed with the further objective of shedding loads so as to prevent depletion of the vehicle battery to a point where the vehicle can no longer be started therewith. Of course, this will render inoperative related vehicle systems, including telematics and keyless entry systems.
Often it is desirable to operate certain vehicle systems or to power external accessories when a vehicle is inactive, for example, operating the vehicle radio or entertainment system, or running small appliances or televisions. However, vehicle batteries will quickly become depleted from such activities. Auxiliary power units have been proposed for providing on-board electrical power in vehicles. Such auxiliary power units may be in addition to or in displacement of traditional belt-driven alternators. It has been proposed that such auxiliary power units would enable operation of all typical vehicle loads—and even some very substantial non-traditional electrical loads such as electrically-powered air-conditioning compressors—during periods of engine-off idle. Such auxiliary power units would need to be scaled to provide significant amounts of electrical energy, and would present cost and packaging challenges.